U.S. Pat. No. 3,947,006 teaches a gas spring of the above-described type, wherein a lock washer is disposed in an annular groove on the outer circumferential surface of the guide piston, and wherein an annular recess is provided in the inside wall of the housing, the recess accepting the lock washer releasably with the piston rod in the extended position. When the piston rod is in its fully extended or withdrawn position, the lock washer engages the recess in the housing wall, locking the piston rod in this position. The piston rod can be pushed into the housing only when this locking force is overcome. The purpose of this feature is to prevent the hatchback or trunk lid or the like of a motor vehicle equipped with such a gas spring from swinging back down because of pressure losses in the gas spring caused by leaks in the seal or by a pressure loss produced by very low temperatures. A particular disadvantage of this known gas spring consists in the fact that the unlocking forces are difficult to determine in advance.
German Offenlegungsschrift No. 23,45,503 likewise teaches a gas spring of the type described above, wherein a mechanical lock is provided, engaging the extended piston rod, to achieve the same purpose, such locking device being formed by a locking cone or a rotating cam lock. This lock is released by pulling the piston rod first slightly out of the housing, then pushing it back into the housing. Hence, when using the hatchback or trunk lid of a motor vehicle, the hatchback or trunk lid must first be raised slightly, then swung down. If these operating instructions are not followed, and the hatchback or trunk lid is closed directly, the gas spring and its lock are destroyed, an obviously undesirable characteristic.